Subscriber access provided by University of Florida | Smathers Libraries
Communication
Spatiotemporal Evolution of Coherent Elastic Strain Waves in a Single MoS Flake 2
Alyssa J. McKenna, Jeffrey K. Eliason, and David J. Flannigan Nano Lett., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Spatiotemporal Evolution of Coherent Elastic Strain Waves in a Single MoS2 Flake Alyssa J. McKenna, Jeffrey K. Eliason, and David J. Flannigan* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455, USA *Author to whom correspondence should be addressed. E-mail:
[email protected] Office: +1 612-625-3867 Fax: +1 612-626-7246
PagePlus 1 ofEnvironment 22 ACS Paragon
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Abstract: We use bright-field imaging in an ultrafast electron microscope to spatiotemporally map the evolution of photoexcited coherent strain waves in a single, micrometer-size flake of MoS2.
Following in situ femtosecond photoexcitation, we observe individual wave trains
emerge from discrete nanoscale morphological features and propagate in-plane along specific wave vectors at approximately the speed of sound (7 nm/ps). Over the span of several-hundred picoseconds, the 50-GHz wave trains (20-ps periods) are observed to undergo phonon-phonon scattering and wave-train interference, resulting in a transition to larger-scale, incoherent structural dynamics.
This incoherent motion further evolves into coherent nanomechanical
oscillations over a few nanoseconds, ultimately leading to MHz, whole-flake multi-mode resonances having microsecond lifetimes. These results provide insight into the low-frequency structural response of MoS2 to relatively coherent optical photoexcitation by elucidating the origin and the evolution of high-velocity, GHz strain waves.
KEYWORDS: ultrafast electron microscopy, structural dynamics, acoustic phonons, transition metal dichalcogenides
PagePlus 2 ofEnvironment 22 ACS Paragon
Page 2 of 22
Page 3 of 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
The properties of extended sheets of MoS2, a heavily-studied transition-metal dichalcogenide (TMD) semiconductor, have been shown to be dependent on both layer number and structural morphology.1-7 This has led to the development of methods aimed specifically at tuning and controlling its electronic, optical, and mechanical responses.8,9
Among these
methods, elastic deformation is particularly effective and versatile, stemming (in part) from widely-varying linear-elastic tensor values and highly direction-dependent transport properties, both of which are directly linked to the structurally-anisotropic bonding in the layered lattice type. For example, application of a tensile, compressive, or shear stress, or a substrate-induced strain, has been shown to alter the electronic and phononic band structures, as well as the effective charge-carrier masses, of few- and single-layer specimens.4,6,10-20 Additionally, the elastic constitutive properties, resonant responses, and phonon-transport behaviors (e.g., Young’s modulus, piezoelectricity, thermoelectricity, etc.) have attracted attention for energy-harvesting and energy-conversion applications, wherein the number of layers plays a distinct role owing to the effect on crystal symmetry.21-28 In addition to direct-contact manipulation, optical excitation offers a means to (remotely) modulate and control the structural and transport properties of MoS2, either through the charge carriers or via the resulting lattice response.29-31 Accordingly, development of a comprehensive view of the time-dependent response of MoS2 to optical excitation – in addition to advancing fundamental understanding – could potentially influence the application space. While chargecarrier and exciton dynamics of MoS2 have been extensively studied with ultrafast spectroscopy,32-43 the concomitant photoexcited low-energy lattice dynamics (e.g., lowfrequency acoustic-phonon excitation and launch, transient thermoelasticity, optomechanical oscillation, etc.) have received far less attention. Indeed, most studies within this particular
PagePlus 3 ofEnvironment 22 ACS Paragon
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
parameter space have focused on understanding relatively low-frequency interlayer shear and breathing modes (~1 THz) using time-averaged Raman spectroscopy.44-47 Importantly, evolution of relatively low-energy coherent structural dynamics having MHz to GHz frequencies is particularly amenable to study with ultrafast electron and X-ray scattering techniques. This is due, in part, to such techniques being sensitive to atomic-scale vibrations, nanoscale elastic deformation, and mesoscale mechanical motion.48-54 Specifically with respect to TMDs, most studies employing such methods have focused on materials that exhibit chargedensity waves (e.g., TaSe2 and TaS2),55-61 though more-recent work has begun to focus on the semiconductors.52,62
For MoS2, Lindenberg and co-workers used MeV ultrafast electron
diffraction to study the reciprocal-space dynamics of a monolayer specimen in a parallel-beam configuration.63 In this way, they were able to measure picosecond in-plane photoinduced wrinkling, electron-phonon coupling times, and rates of thermal-energy transfer to a substrate. To date, however, the spatiotemporal evolution of photoexcited coherent elastic strain waves in MoS2 – spanning picoseconds to microseconds (GHz to MHz) and nanometers to micrometers – has not been reported. Here, we used an ultrafast electron microscope (UEM)64-66 to directly image the spatiotemporal dynamics of photoexcited propagating strain waves in individual, micrometersize multilayer flakes of MoS2 (
